The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Riikka Puurunen

Dissertation for the degree of Doctor of Science in Technology to be presented
with due permission of the Department of Chemical Technology for public
examination and debate in Auditorium Ke 2 (Komppa-sali) at Helsinki University
of Technology (Espoo, Finland) on the
25th of October, 2002, at 12 o'clock noon.

Abstract

Catalyst supports with novel chemical and physical properties are needed for
producing new families of catalytic materials. The goals of this work were to
demonstrate the preparation of aluminium nitride type materials and evaluate
their properties as catalyst supports.

To obtain aluminium nitride in high-surface-area form suitable for catalyst
applications, porous silica and alumina supports were surfaced with aluminium
nitride by the atomic layer deposition (ALD) technique by repeating the
separate, saturating reactions of gaseous trimethylaluminium (TMA) and
ammonia. The reaction temperatures of TMA and ammonia were 150 and
550 °C, respectively. Six reaction cycles led to an average growth of
2.4 aluminium atoms per cycle per square nanometre, and, according to
low-energy ion scattering, 74% coverage. The aluminium nitride species were
shown to be evenly distributed on the silica. The aluminium nitride appeared
amorphous in X-ray diffraction, but 27Al nuclear
magnetic resonance (NMR) spectroscopy confirmed its formation.

Insight into the growth mechanism of aluminium nitride was obtained by
investigation of the individual steps leading to the growth. The surface
reaction products after the TMA and ammonia reactions were identified by
diffuse reflectance Fourier transform infrared spectroscopy and
1H, 13C and
29Si NMR, and they were quantified by elemental
analysis and 1H NMR. TMA reacted through ligand
exchange with hydrogen atoms present on the surface in hydroxyl groups (OH)
and amino groups (NHx), releasing methane,
and further through dissociation in siloxane bridges, coordinatively
unsaturated aluminium-oxygen pairs and nitrogen bridges. Steric hindrance
imposed by the methyl ligands defined the saturation of the surface with
adsorbed species; at saturation, there were five to six methyl groups per
square nanometre. The ammonia reaction replaced the methyl groups present on
TMA-modified surfaces with NHx groups
(x = 2, 1 or 0). The results for the TMA reaction agreed
quantitatively with the results obtained by others for the ALD growth of
aluminium oxide thin films. A model was derived that relates the size and
reactivity of the metal reactant to the growth per cycle of the oxide by ALD.

The properties of the AlN/oxide materials as catalyst supports were evaluated
for cobalt hydroformylation and chromium dehydrogenation catalysts. In the
preparation of the catalysts from cobalt(III) and chromium(III)
acetylacetonate by ALD, the factor defining the saturation of the reaction was
the same as on the respective oxides: the steric hindrance imposed by the
acetylacetonate (acac) ligands. Dissociative and associative reactions of the
metal acetylacetonate reactants and of the Hacac released in ligand exchange
reaction took place on the AlN/oxide supports. This was a disadvantage for the
Co/AlN/silica catalysts, as the high acac/Co ratio of the surface complex led
to the desorption of cobalt(II) acetylacetonate during catalytic testing in
hydroformylation.

After removal of the remaining acac ligands with ammonia, the activity of the
Cr(III)/alumina and Cr(III)/AlN/alumina catalysts was evaluated in isobutane
dehydrogenation at 580 °C. The aluminium nitride modification of the
support decreased the dehydrogenation activity of the chromium catalysts.
Pairs of chromium and oxygen ions seem to be required for active chromium
catalysts, and replacing the neighbouring oxygen with nitrogen was a
disadvantage.

Although no improvements were observed in catalytic performance of the
prepared catalysts relative to conventional systems, the information obtained
will be useful in the future investigations of the growth of aluminium nitride
and of other materials by ALD and in the identification of the active sites on
dehydrogenation catalysts.

This thesis consists of an overview and of the following 7 publications: